An unknown object apparently collided with a satellite with NASA Langley Research Center connections in November, sending several broken pieces flying into orbit.

The satellite was decommissioned in December 2005, but was one of NASA's largest and brightest in low-Earth orbit, and popular among amateur sky watchers. Called UARS, Upper Atmosphere Research Satellite, it was launched in 1991 and carried a NASA Langley instrument called HALOE that captured data about the chemistry of the atmosphere.

On Nov. 10, something apparently hit the school bus-sized orbiter. It could have been one of the many pieces in a growing field of "space junk." It could have been a meteoroid. Space debris often leaves pings and dents in satellites and even the space shuttle, and aging satellites decay over time. But a collision that actually creates new pieces of debris is more rare.

"When I heard this, I was shocked," said Jim Russell, a Hampton University professor who was the project lead for HALOE. "This is very unexpected. That's not normal decay."

Nicholas L. Johnson, chief scientist for NASA's Orbital Debris Program, said it remains unclear what happened to UARS. Four pieces bigger than 4 inches in diameter - roughly the size of a trackable piece of space junk - were sent into orbit, but it is unclear how large those pieces are.

A collision from a meteoroid or another piece of debris is the best hypothesis, Johnson said. The core of the spacecraft appears to still be intact.

"Unfortunately, we might not ever learn what caused the event," Johnson said.

Mark Matney, who works with Johnson in the Orbital Debris Program, said satellites with still-functioning pressurized systems sometimes eject new debris if a tank explodes. But UARS had no such systems, so a collision is the best explanation.

Only three known collisions between two satellites have ever occurred, Matney said. But these "anomalous" events, where it's not clear what one of the colliding objects was, do happen occasionally, he said.

"It's very hard to determine," what might have hit UARS, Matney said.

Decommissioned satellites typically continue to orbit for years before losing energy and falling toward Earth, usually to burn up. UARS was expected to fall out of orbit around 2011, Russell said.

UARS weighed about 13,000 pounds and measured 35 feet long and 15 feet wide, perhaps making it a better target than most.

The U.S. Space Surveillance Network, run by the Air Force, estimates there are about 10,000 objects in low-Earth orbit that are larger than 4 inches. The network's sensitive ground-based instruments can track those objects. The objects range from communication satellites to the International Space Station to junk - pieces of decaying satellites and the remnants of rocket boosters.

The field of debris has become an increasing concern as the number of pieces continues to increase. The debris field was also significantly expanded in January of this year, when China angered the U.S. and other nations by testing an anti-satellite missile. The Chinese destroyed their FY-1C satellite, an aging weather observer.

The missile test exploded the satellite into more than 1,000 pieces of debris, and U.S. intelligence and defense analysts almost immediately deduced what had happened, before the Chinese government admitted it a few weeks later.

The UARS collision created only a handful of new pieces of debris, but still, "You hope it's not anything sinister," said Ellis Remsberg, a Langley scientist who worked on HALOE with Russell.

Two of the "large" pieces that broke off UARS have apparently already burned up in the atmosphere, Johnson said. The other two pieces will likely do the same.

What remains of the craft's core will continue to orbit for some time - barring another collision.

"It was a bright object in the sky," said Russell. "You could see it over the Peninsula every 33 days."

What is UARS?

The Upper Atmosphere Research Satellite was launched in 1991 and carried multiple atmospheric-observing instruments, one of which was built at NASA Langley Research Center in Hampton. The satellite, which weighed about 13,000 pounds and was more than 30 feet long, was decommissioned in 2005 but continued to orbit Earth. It was apparently struck by an object in space in November.